The present invention relates to microdialysis probes, in particular a microdialysis probe for measuring the concentration of a dissolved substance in a tissue.
Microdialysis probes generally comprise a probe body and an injection needle as probe needle for introduction into, for example, a human or animal tissue, the probe needle being completely surrounded by the tissue. The probe needle comprises an inlet line (or conduit) and an outlet line for a perfusion solution. Moreover, in one area of the line for the perfusion solution, a dialysis membrane is arranged to be in contact with the tissue environment. Between the tissue environment and the perfusion solution, the concentration of permeable substances dissolved in the tissue is equalized along the membrane surface.
Conventional microdialysis probes typically have, for example, a coaxial structure. Two hollow cylinders are arranged one inside the other, the inner hollow cylinder serving as an inlet line for the perfusion solution which, at a transition area in a distal end portion of the probe needle, passes into the area between the inner cylinder and the outer cylinder and is conveyed back within this annular channel. The dialysis membrane can, for example, be arranged in a front area where the perfusion solution turns back or, alternatively, it can form part of the outer hollow cylinder. Microdialysis probes of this type have the disadvantage that the perfusion solution comes into contact with the tissue environment only across a small membrane surface and, at most, across the length of the probe needle, and that a membrane hollow fiber acting as outer cylinder is exposed to the pressure of the tissue, and the outlet line can therefore become blocked. Moreover, in the case of a short probe needle, there is often not enough time left to obtain a complete concentration equalization. By contrast, a long probe needle is unpleasant for the user. With a straight return of the perfusion solution, the flow velocity is too high to permit all of the desired concentration equalization.
DE 199 37 099 A1 discloses microdialysis probes in which the inlet line and outlet line for a perfusion solution are arranged next to one another. For this purpose, two mutually adjacent tubes, for example, are provided which have a flow transfer area for the perfusion solution. It is also possible to arrange, inside a microdialysis membrane in the form of a hollow fiber, a support structure which divides the hollow fiber into different hollow channels, again with the possibility of a flow transfer between the channels being provided. Here too, the inlet line and outlet line have a straight course, with the result that concentration equalization is, in some cases, not optimal.